Audio devices which are used, for example, in car radios, generally allow a user to set various volume levels so as to vary the intensity of the sound produced by one of more loud-speakers connected thereto. Typically, this variation is achieved by means of a volume-control unit which attenuates (or amplifies), dependent on the volume level set, an audio signal input to the device.
These audio devices usually also allow the user to switch on (or off) a loudness effect which improves the physiological sensation produced by the sound. This effect typically includes an amplification of the low-frequency components of the audio signal by a predetermined value (the high-frequencies possibly also being amplified by a lower value) so as to compensate for the lower response of the human ear to low-frequency sounds. In some known audio devices, the loudness effect amplifies the low-frequency components of the audio signal by a value which depends on the volume level set. More particularly, the loudness effect may be accentuated for low volume levels so as to compensate for the poorer response of the human ear to low frequencies when the sound is of lower intensity.
The loudness effect is achieved by means of a loudness-control unit comprising a low-pass filter which, when the unit is in an active state, attenuates the components of the audio signal having frequencies greater than a cut-off frequency, for example, of 1 kHz by a predetermined value (possibly also attenuating the other components by a lower value). When the loudness-control unit is in an inactive state, it attenuates every component of the signal by this value. If the loudness effect is switched off by the user, the loudness-control unit is put in the inactive state and the attenuation value is set at zero (0 dB) so that it does not modify the audio signal. When the user switches on the loudness effect, the audio device obtains the amplification value for the low-frequency components corresponding to the volume level set and consequently activates the loudness-control unit so as to attenuate the high-frequency components of the audio signal by an equal value. To achieve the desired amplification effect on the low-frequency components, it is then necessary to reduce correspondingly the attenuation of the audio signal produced by the volume-control unit. Similar operations are carried out in reverse order when the user switches the loudness effect off.
A disadvantage of the known devices described above is that the control of the loudness effect is quite complex and makes fairly high processing demands on the audio device.